Project Summary Herpes simplex virus 1 (HSV-1) encephalitis (HSE) is the most common sporadic viral encephalitis in Western countries. Forebrain HSE (which develops via olfactory neurons) in otherwise healthy children can result from inborn errors of the TLR3 pathway (mutations in TLR3, UNC93B1, TRIF, TRAF3, TBK1, and IRF3), whereas brainstem HSE (via trigeminal (TG) neurons) can result from inborn errors of RNA lariat metabolism (DBR1). Children with a broader defect of impaired production of (NEMO) or response to all IFNs (STAT1) are prone to HSE and other infections. We analyzed the cellular basis of HSE by deriving peripheral and central nervous system (PNS and CNS) cells from induced pluripotent stem cells (iPSC) (NIH R01NS072381). TLR3-deficient forebrain cortical neurons and oligodendrocyte precursors have impaired anti-HSV-1 cell-intrinsic immunity, unlike astrocytes and neural stem cells; microglial cells were not tested. Moreover, iPSC-derived TG neurons do not rely on TLR3 to control HSV-1; olfactory neurons were not tested. Finally, TLR3 controls both basal IFN levels and early steps of anti-HSV-1 immunity in cortical neurons. Childhood HSE results thus from inborn errors of non-hematopoietic, CNS-specific, cell-intrinsic immunity, affecting cortical neurons and oligodendrocytes in particular. We have since identified new forebrain HSE-causing genes that are connected to the TLR3-IFN circuit (MEX3B, IFNAR1), the TLR3-necroptosis pathway (RIPK1, RIPK3), and of unknown function (SNORA31, TMEFF1). In this renewal application, we hypothesize that genetic etiologies of forebrain HSE impair intrinsic immunity in cortical but not olfactory neurons, and in oligodendrocytes and probably microglial cells, and DBR1 deficiency impairs intrinsic immunity in brainstem and/or TG neurons and probably microglial cells. First, we will assess the responses of iPSC-derived olfactory and cortical neurons, and other CNS cells, from controls and patients mutated in new forebrain HSE-causing genes, to TLR3, IFN-?/? or IFN-?, and HSV-1 stimulation. Second, we will study RNA lariat accumulation, immunity to HSV-1 infection, and responses to IFN-?/? or IFN- ?, in control and DBR1-deficient iPSC-differentiated TG and brainstem neurons. Third, the role of microglial cells in forebrain vs brainstem HSE will be assessed by studying the responses to TLR3 agonists, IFN-?/?, IFN-? and HSV-1, in isolation and in neuronal co-culture. Patient and isogenic iPSC lines in which the mutation is corrected or introduced by gene editing will be used. Cell-intrinsic immunity to HSV-1 and its molecular basis will be analyzed. Exciting preliminary data have been obtained, including (1) novel genetic etiologies of HSE, (2) novel mechanisms by which TLR3 controls HSV-1 in cortical neurons, and (3) novel protocols to differentiate brainstem neurons and microglial cells. The expand of this human iPSC-based study of HSE will enable us to dissect in greater breadth and depth its molecular and cellular basis in children with inborn errors of CNS-intrinsic immunity to HSV-1.